When typical raw wastewater arrives at the treatment plant it may be septic (odorous) and contain dissolved gases, greases, oils, rags, plastics of all sorts, grit, rocks, wood particles and a variety of other undesirable materials. Therefore pre-conditioning, or pre-treatment, is essential. Traditionally, preconditioning is accomplished by individual screening and grit removal devices, arranged in series. Scum is removed at primary or secondary clarifiers where it is collected and added to biological sludge from the main treatment process, and is a constant source of day-to-day operating problems. This technology also fails to prevent rags, and other fibrous material from finding their way into the main treatment units where they clog pumps and cause other problems. Normally these individual units are placed close to the plant inlet, where, in anticipation of future plant expansion, they occupy a large area. Also it is often necessary to house some units, depending on the climatic conditions and the particular equipment that has been selected.
This approach is costly in terms of initial plant design and construction costs, and also in terms of on-going operating costs. Operation of a plant of this type is fairly labor-intensive in that the individual units often require personal supervision to ensure proper operation. It is very difficult to automate or even partially automate a sewage treatment plant designed on these principles.
Flow rate measurement normally is performed at a fairly early stage in the treatment process in order to provide a means of monitoring the rate of sewage flow through the plant in comparison with the design capacity of the plant. Typically, flow rate is measured immediately after the fine screening, by passing the wastewater through a parshall flume having a restricted cross-section which causes the liquid level to elevate. By measuring the liquid level within the restricted cross-section, it is possible to measure the flow rate. The difficulty is that, at this stage in the process, the wastewater still contains scum and other debris, as well as bacteria that may tend to cause the flume become odorous.